19. Lewis structures and VSEPR

Question 1

What is the octet rule?

Answer 1

To achieve stability each atom in a molecule shares electrons to have 8 electrons in its valence shell

Applies to all elements except H

Has many exceptions

Question 2

What are the exceptions to the octet rule?

Answer 2

• Radicals (stable): CH₃ (unstable), NO (stable)
• Electron poor molecules: BF₃ ( when with NH3 - shared e- donor-acceptor complex, dative bond)
• Hypervalent molecules: PCl₅ (exceeds octet but stable)

2 group non-metals have to obey the octet, others can disobey the octet

Question 3

What is formal charge and how it is determined?

Answer 3

Formal charge - charge on an atom in molecule - the difference between the # of valence e in free atom and # of e assigned to that atom in Lewis structure

Question 4

Explain valance shell electron pair repulsion theory (VSEPR)

Answer 4

Question 5

Why does the molecular shape matter?

Answer 5

Question 6

What are the possible shapes of molecules?

Answer 6

Question 7

How are mutiple bonds treated in determining molecular shape?

Answer 7

Question 8

How is resonance accounted in molecular shapes?

Answer 8

All bonds become the same → equal distribution of bonds instead of squishing as in multiple bonds

Question 9

What are the limitations of VSEPR?

Answer 9

Question 10

What is the maximum coordination number and how it is calculated?

Answer 10

_Coordination numbe_r - of the central atom when it is bounded to ligands in complex ions,

Maximum coordination number - depends on the sizes of the central atom → the larger the atom - the more ligands it can bind

Ex: NF₅ does not exist but PF₅ exists because N atom is too small to form a complex with more than 4 ligands, so NF₅ cannot exist (N atom < P atom)